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A digraph is represented as DG vs es, where vs is the list of vertices, and es is the list of edges.
Edges are directed: an edge (u,v) means an edge from u to v.
A digraph is considered to be in normal form if both es and vs are in ascending order.
This is the preferred form, and some functions will only work for digraphs in normal form.

Given a directed acyclic graph (DAG), return a canonical representative for its isomorphism class.
isoRepDAG dag is isomorphic to dag. It follows that if isoRepDAG dagA == isoRepDAG dagB then dagA is isomorphic to dagB.
Conversely, isoRepDAG dag is the minimal element in the isomorphism class, subject to some constraints.
It follows that if dagA is isomorphic to dagB, then isoRepDAG dagA == isoRepDAG dagB.

The algorithm of course is faster on some DAGs than others: roughly speaking,
it prefers "tall" DAGs (long chains) to "wide" DAGs (long antichains),
and it prefers asymmetric DAGs (ie those with smaller automorphism groups).